This invention relates to an intake air quantity control method for internal combustion engines, and more particularly to a method of this kind which is adapted to supply the engine with a required amount of intake air at transition of engine operation from a condition wherein fuel cut is to be effected to a normal condition wherein the fuel supply to the engine is to be effected, to thereby prevent engine stall.
It is generally known that the mass of air sucked into each cylinder of the engine per suction stroke thereof is a function of the valve opening of the throttle valve, the engine rotational speed, the configuration of the intake pipe of the engine, and other parameters, and accordingly, it varies in response to changes in these parameters. Also, as generally known, the mass of air sucked into an engine cylinder per suction stroke is represented in terms of the charging efficiency of the engine.
When the engine is operating in a low speed region with the clutch disengaged to disconnect the output shaft of the engine from a driven shaft coupled to the wheels of a vehicle and also with the throttle valve fully closed, the engine is supplied with a quantity of intake air corresponding to the charging efficiency of the engine which is substantially equal to a quantity of intake air required by engine loads which are determined by the frictional resistance of sliding parts of the engine, etc., so long as the engine speed remains in the vicinity of its idling rpm, thereby maintaining the engine speed substantially constant. However, when the engine is operating in a high speed region with the clutch disengaged and the throttle valve fully closed, the charging efficiency of the engine becomes too small to maintain the engine speed as it is, resulting in a sudden drop in the engine speed.
Such sudden drop in the engine speed can frequently occur particularly in an internal combustion engine equipped with an electronic fuel injection system at fuel-cut operation which is usually effected at deceleration of the engine with the throttle valve fully closed. Therefore, there exists a fear of engine stall at the transition of engine operation from a condition wherein fuel cut is to be effected to a condition wherein the fuel supply to the engine is to be effected. In order to prevent such engine stall, the charging efficiency of the engine has to be increased.
In some internal combustion engines equipped with electronic fuel injection systems, the internal volume of the intake pipe at a location downstream of the throttle valve is enlarged so as to reduce fluctuations of pressure prevailing in the intake pipe, thereby eliminating changes in the charging efficiency of the engine. However, such increase in the internal volume of the intake pipe causes a time lag in the control of the intake air quantity. More specifically, even if means for increasing the quantity of intake air is actuated immediately upon detection of a transition of the engine operation from a fuel cut effecting condition to a fuel supply requiring condition, there exists a time lag between the time of actuation of the intake air quantity increasing means and the time a required increased quantity of intake air is actually sucked into the engine cylinders, depending upon the configuration, length, etc. of the intake pipe applied. If the time lag is large, engine stall can also occur, as stated before.